Inhibitory effects of the essential oil of Mentha pulegium on the isolated rat myometrium

The effects of the essential oil of Mentha pulegium L. (EOMP), a plant commonly known as "pennyroyal" or "poejo" that is used in folk medicine as an abortifaceant, were assessed on the isolated rat myometrium. Myometrial strips were stimulated with 10 nM oxytocin or 10 mu M PGF(2 alpha). EOMP (10 - 300 mu g/mL) concentration-dependently and reversibly inhibited the amplitude of oscillatory contractions, being approximately 3-fold more active against contractions stimulated by oxytocin than those by PGF(2 alpha) (IC50 values of 45.7 +/- 5.6 mu g/mL and 160.9 +/- 5.9 mu g/mL, respectively), although the maximal inhibitory effect occurred at the same concentration (300 mu g/mL) in both cases. This action was shared by pulegone (30-300 mu M), the principal component of the essential oil (IC50 values of 21.8 +/- 2.1 mu g/mL and 12.7 +/- 4.6 mu g/mL, respectively). Nifedipine (30 nM - 30 mu M) also abolished agonist-stimulated contractions, and was approximately twice and 12 times as potent as EOMP in inhibiting oxytocin- and prostaglandin F-2 alpha (PGF(2 alpha))-stimulated contractions, respectively. In conclusion, our results show that the essential oil of the abortifaceant plant Mentha pulegium exerts an inhibitory effect on the contractile activity of the isolated rat myometrium. This oil shares a common effect with the voltage-dependent calcium channel (VDCC) blocker nifedipine, although ostensibly acting via a different mechanism. It thus appears that EOMP and pulegone do not exert direct toxic effects on the myometrium per se that would cause abortion, and other possibilities such as systemic metabolism of plant constituents may rather underlie the abusive use of Mentha pulegium in popular medicine.